Dual cell refining of silicon and germanium



Nov. 23, 1965 R. MONNIER ETA]. 3,219,551

DUAL CELL REFINING 0F SILICON AND GERMANIUM Filed March 14, 1962 INVENTORS:

8 R Y mm a N W N C m M5 T R M/ A R R EW l AY E OLB H RD T United States Patent M 3,219,561 DUAL CELL REFlNlNG 0F SILICON AND GERMANIUM Robert Monnier and Dlawar Barakat, both of Geneva,

Switzerland, assignors to The General Trustee Comparry Inc., Geneva, Switzerland Filed Mar. 14, 1962, Ser. No. 179,726 12 Claims. (Cl. 204-60) This invention relates to the electrolytic production and refining of metals particularly of silicon and germanium, and, more particularly, to a new electrolytic method of producing these elements in a high purity state from their oxides.

Electrolytic methods of preparing silicon and germanium have been proposed heretofore. Nevertheless, so far none of these methods have been utilized in the industry for several reasons; among which are the low purity of the elements obtained rendering them unsuitable for further refining by zone-refining or the like and diflicult operating conditions involved in their production.

In accordance with the present invention, we have discovered that it is possible to obtain very pure silicon from silica and to obtain very pure germanium from germanium oxide by a double electrolysis conducted in a cell of suitable type wherein the oxide is first reduced to a standard pure metal in a first compartment and then is refined and deposited on a cathode with a high purity capable of exceeding 99.99% in a second compartment. More particularly, the electrolysis is conducted with one or two fused salt baths or electrolytes of any of the following type: (1) an alkali metal cryolite alone or a mixture of different alkali metal cryolites with or without an alkali metal or alkaline earth metal fluoride or a mixture of the same; (2) alkaline earth or alkaline metal fluorides alone or in admixture; (3) the same as in (l) and (2) with oxides of the metal or element being refined, and (4) alkali metal and/or alkaline earth metal chlorides in admixture with alkali metal or alkaline earth fluorides or fluosilicates. In such electrolytes, the chlorides alone are not satisfactory for the reason that they form volatile compounds with silicon, for example, which evolve at bath temperature. The same or different electrolyte can be used in the two compartments of the cell. Working with the latter conditions is very advantageous. In the first compartment, an electrolyte can be used which is capable of dissolving a certain quantity of the oxide of the metal to be deposited, such as a bath based on cryolites. In the second compartment any of the above mentioned electrolytes can be used. An electrolyte containing alkaline fluosilicates and chlorides without cryolites is particularly useful because the deposited metal is easily recovered from the bath carried over with it.

The cell in which a reduction and refining takes place may be formed of material capable of withstanding bath temperatures on the order of 500 to 1100 C., the temperature at which the electrolyte is molten, and is characterized by the presence of a dividing partition therein which extends toward the bottom of the cell and forms an anode compartment and a cathode compartment which communicate with each other through a passage between the bottom of the cell and the lower edge of the partition. A pool of molten alloy of the metal being produced fills the cell to a level above the lower edge of the partition and thereby separates an overlying molten bath of the electrolyte in the anode compartment from a molten bath of electrolyte in the cathode compartment. An anode and a cathode are immersed in the electrolytic baths in the respective compartments but are out of direct contact with the molten metal alloy in the bottom of the cell.

An oxide of silicon or germanium is introduced into 3,219,551 Patented Nov. 23, 1965 or present in the electrolyte in the anode compartment and when a direct current of a current density of a proper value, e.g. 5 to amperes per square decimeter, is applied across the anode-cathode circuit, the metal of the oxide is deposited into molten alloy at the bottom of the cell. At the same time, silicon or germanium in the alloy, by the eflect of current, goes into the solution, migrates therefrom and is deposited on the cathode in a highly refined state. By mounting the cathode in such a manner that it can be removed or replaced readily, the deposit of refined and highly purified metal thereon can be removed and separated from any electrolyte clinging to the deposit and to the cathode. The metal recovered from the cathode has a sufiiciently high purity, e.g. 99.99+% that it can be used directly for many purposes requiring high purity silicon or germanium, particularly, it is very suitable to be purified by zone-refining.

For a better understanding of the present invention, reference may be had to the accompanying drawing in which the single figure discloses a typical cell by means of which an oxide of silicon or germanium can be reduced to metal and deposited in a highly refined state on the cathode of the cell. As shown in the drawing, the cell 9 comprises a steel casing or jacket 10, in which the refractory bricks are laid in such a way as to form a lining which is coated with material resistant to corrosion under operating conditions, e. g. carbon or graphite but preferably a poor electrically conductive material such as SiC bonded with silicon nitride. Any suitable means may be used for heating the cell, such as electric resistance heaters, but preferably the cell is heated only by the current flowing between the anode 11 and the cathode 12.

Extending across about the middle of the cell 10 is a partition 13 having a lower edge spaced from the bottom of the cell to provide a passage 14 permitting circulation of a molten alloy 15 between the anode compartment 16 and the cathode compartment 17. The partition wall 13 may be composed of carbon, graphite or refractory bricks covered with silicon carbide bonded with silicon nitride of the type of Crystolon. The partition 13 must be made of a material which does not conduct electricity and is resistant to the corrosion by the electrolytes with which it is in contact.

Circulation of the alloy is important as will be explained hereinafter and may be accomplished by any of a number of dilterent means, such as by rocking the cell, agitation and the like. A typical agitator includes a shaft 18 extending downwardly through the partition 13 and rotated at slow speed by means of a motor and interposed reduction gearing 19. A rod-like stirrer 20 is mounted transversely on the lower end of the shaft 18 below the partition 13. The stirrer may be composed of graphite, carbon or the like and may be reenforced internally, if required. That portion of the stirred shaft 18 which is not immersed in the molten bath may be protected from oxidation by means of a silicon carbide tube or the like, not shown.

A graphite or carbon anode 11 and a graphite or carbon cathode 12 are mounted for movement into and out of their respective compartments 16 and 17 in order to permit removal and replacement. In particular, the cathode 12 should be removable to enable the deposit to be separated therefrom.

Baths of suitable molten salt electrolytes 21 and 22 of the type described above are superimposed on the molten alloy 15, and immerse the lower ends of the anode and cathode which are spaced from the molten alloy 15. A typical example of the method as practiced with the apparatus is as follows.

Example 1 The anode and cathode 11 and 12 of the cell have an area of 50 square decimeters each. Each of the anode 16 and cathode 17 compartments has a bottom area of 100 square decimeters. Into the cell is poured a molten alloy of copper and silicon containing 16% silicon. Enough of the molten alloy is charged into the cell to fill it above the level of the lower end or edge of the partition 13. Compartments 16 and 17 are charged with a molten electrolyte composed of sodium cryolite containing 4% silica. With the electrodes 11 and 12 immersed in the electrolyte, a direct current of an intensity of about 2000 amperes, i.e., a current density of 40 amperes per decimeter is passed between the electrodes. The stirrer 18, 20 is rotated slowly. A temperature of about 1000 C. is maintained in the cell by regulating the distance between the anode and the cathode or by regulating the intensity of the current. Pure quartz powder is supplied to the anode compartment 16 to maintain the silica content of the electrolyte therein at between 1% and 4%. By virtue of the action of the stirrer, a steady movement of the molten anode-cathode alloy 15 occurs, with the result that silicon present in the alloy migrates to the cathode compartment 17 and is discharged from the alloy and deposited on the cathode 12. When a sufficient amount of silicon is deposited on the cathode 12, the latter is withdrawn from the cathode compartment 17 and the deposit is removed from the cathode. The material removed from the cathode is broken up into small pieces and then is extracted by immersing the pieces in a 13% solution of aluminum chloride until the electrolyte is dissolved. The residue containing silicon is washed with hydrofluoric acid followed by washing with hydrochloric acid and is then filtered. In the process described, the silicon crystals separated from the filtrate had a purity of 99.99% The current efirciency of the operation was 75%.

. Example 2 In a similar manner, the cell was charged with molten silver-germanium alloy containing 20% germanium to form the anode-cathode layer at the bottom of the cell. A mixture of 70% sodium cryolite, 28% sodium fluoride and 2% germanium oxide was poured into the anode compartment. A molten mixture of sodium fluoride and potassium fluoride in substantially equal proportions by weight was poured into the cathode compartment. The cell temperature was maintained at about 930 C. by passing an electric current of a density of 2000 amperes between the anode and the cathode, that is, a current density of 40 amperes per square decimeter. Germanium oxide was supplied to the anode compartment to maintain a concentration thereof between about 1% land 2% in the electrolyte.

Germanium deposited on the cathode, after removal of soluble components by pulverizing the deposit and Washing with boiling dilute hydrochloric acid, and filtering was 99.99% pure. The cell operated at a current efiiciency of 60%.

Similar results are obtained in refining germanium by using a copper-germanium alloy as the molten anode-cathode in the bottom of the cell.

The proportions of the components of the anode-cathode molten alloy can be varied so long as the alloy is molten at cell operating temperatures. For example, germanium alloys with nobler metals than germanium for example Ag or Cu may contain up to 50% germanium and are molten at temperatures less than 1000 C.

The structure of the cell, the current density and the electrolytes are susceptible to variation as indicated. Refined metals obtained in accordance with the invention may be used for purposes commensurate with their purity. For example, 99.99% pure silicon or germanium obtained in the manner disclosed in the examples can be used in many different fields, and if higher purity than 99.99% is required, they can be purified further, for example, by zone-refining.

Accordingly, it will be understood that the method, apparatus and examples described herein are illustrative.

We claim:

1. A method of producing refined silicon and germanium comprising passing a direct current between an anode in contact with a molten salt bath containing a fluoride and an oxide of a metal of the class consisting of germanium and silicon and a cathode in contact with another molten salt bath containing a fluoride, said baths being separated by a molten alloy of the metal corresponding to said oxide and a nobler metal to reduce said oxide to metal and deposit it on said cathode.

2. The method set forth in claim 1 in which said oxide is silica and said metal corresponding to said oxide is silicon.

3. The method set forth in claim 1 in which said oxide is germanium oxide and said metal corresponding to said oxide is germanium.

4. The method set forth in claim 1 in which said baths are different in the two compartments.

5. The method set forth in claim 1 in which said salt baths are the same in the two compartments.

6. The method set forth in claim 1 in which said salt bath contains a cryolite.

7. The method set forth in claim 1 in which said molten alloy is circulated in contact with said baths while direct current is passed between said anode and cathode through said molten salt baths and said molten alloy.

8. A method of producing refined silicon comprising passing a direct current between an anode and a cathode, each being in contact with separate molten salt baths containing a fluoride, said salt baths being separated by and in contact with a molten alloy of silicon and a metal nobler than silicon, said alloy containing less than 50% silicon, maintaining at least 1% silica in the salt bath in contact with said anode for reduction to silicon and deposit thereof on said cathode.

9. The method set forth in claim 8 comprising circulating said molten alloy between said salt baths and maintaining said salt baths and said alloy above their melting points.

10. The method set forth in claim 8 in which the said salt bath of the anodic compartment contains sodium cryolite and silica and the said salt bath of the cathodic compartment contains alkali chlorides and potassium fiuosilicates.

11. A method of producing refined germanium comprising passing a direct current between an anode and a cathode, each being in contact with separate molten salt baths containing a fluoride, said salt baths being separated by and in contact with a molten metal containing germanium, maintaining at least 1% of germanium oxide in said salt bath in contact with said anode for reduction to germanium and deposit thereof on said cathode.

12. The method set forth in claim 11 comprising circulating said molten metal between said salt baths and maintaining said salt baths and said metal above their melting points.

References Cited by the Examiner UNITED STATES PATENTS 800,984 10/ 1905 Chance 204-60 2,861,030 11/1958 Slatin. 2,892,763 6/1959 Stern et al 20460 2,952,605 9/1960 De Varda 204-243 3,009,870 11/ 1961 Helling et a1 204243 3,030,284 4/1962 Stern 20460 FOREIGN PATENTS 533,780 5/1955 Belgium.

JOHN H. MACK, Primary Examiner.

JOHN R. SPECK, WINSTON A. DOUGLAS,

Examiners. 

1. A METHOD OF LPRODUCING REFINED SILICON AND GERMANIUM COMPRISING PASSING A DIRECT CURRENT BETWEEN AN ANODE IN CONTACT WITH A MOLTEN SALT BATH CONTAINING A FLUORIDE AND AN OXIDE OF A METAL OF THE CLASS CONSISTING OF GERMANIUM AND SILICON AND A CATHODE IN CONTACT WITH ANOTHER MOLTEN SALT BATH CONTAINING A FLUORIDE, SAID BATHS BEING SEPARATED BY A MOLTEN ALLOY OF THE METAL CORRESPONDING TO SAID OXIDE AND A NOBLER METAL TO REDUCE SAID OXIDE TO METAL AND DEPOSIT IT ON SAID CATHODE. 